Drive Engineering - Practical Implementation - Volume 7

....

. . .. . .. . .. . .. . .

........

........

........

........

........

........

........

........

........

....

....

52525253

Introduction

Introduction

Developments in the fields of electronics and materials processing have changed the situation in theworld of drive technology. Servo technology has to date mainly employed DC permanent-magnetmotors. The biggest drawback of AC motors compared to DC motors was their inferior speed controlfeatures. Recent developments in the field of electronics, particularly the microcontroller, mean thatan appropriate control system can now be included to compensate for this.These developments have led to a shift in emphasis in drive systems away from DC towards ACmotors. This trend towards AC synchronous motors is particularly evident in servo systems, whichwere previously almost always implemented using DC technology.New, powerful permanent magnets made of samarium-cobalt and neodymium iron boron canincrease the performance of the motor as a result of their high energy density, while at the same timereducing the mass of the motor. As a result, the dynamic properties of the drive are improved andthe frame size of the motor reduced.

1.1Definition of servo technologyMany applications place high demands on modern drive technology with regard to:

positioning accuracy

speed accuracy

speed control range

torque stability

overload capability

dynamic performance

Demands on the dynamic properties of a drive, in other words its time response, arose as a result ofever faster machining processes, increases in machining cycles and the associated productionefficiency of machines. The accuracy of a drive is very often instrumental in determining for whichapplications a drive system can be used.A modern, dynamic drive system has to be able to satisfy these requirements.A definition of servo drivesServo drives are drive systems that show a dynamic and accurate response over a wide speed rangeand are also capable of coping with overload situations.The word servo comes from the Latin servus, which can be translated as servant, slave or helper.In the machine tool sector, servo drives were primarily auxiliary drives. However, this situation haschanged, so that nowadays main drives are also implemented using servo technology.In this volume, we have used the terms servo drive and dynamic drive to mean one and the samething. They always refer to AC permanent-field synchronous motors and their associated controlsystems.

Drive Engineering - Practical Implementation - Volume 7

Introduction

1.2 Development of servo drives

The term servo drive suggests some sort of auxiliary drive. This may well have been true 40 yearsago in the machine tool industry, when the machines, e.g. lathes, were often still driven by hand.Pneumatic, hydraulic or fixed-speed AC motors were only used where high levels of torque wererequired. It was the skill of the lathe operator and the manual measurements and checks he carriedout that determined how quickly and accurately the workpiece could be machined.On the other side was the main drive, which used pneumatic, hydraulic or electrical means to achievea more or less constant and controlled main spindle speed.

1.2.1 Technical development of servo drives

Initially, hydraulic and pneumatic servo drives dominated the market. The DC drive became moreimportant again in the sixties with the advent of silicon semiconductors.Servo drives were also affected by this development: In view of the requirement for improved dynamicresponse, development started to go in two separate directions.The first saw the required reduction in the mass moment of inertia of the motor in the shape of anextremely short, disc-shaped and iron-free rotor. The second concentrated on a long, very thin rotor.In both instances, the start of the seventies saw permanent magnets being used instead of excitationwindings. This meant torque was produced more quickly and resulted in greater efficiency.What type of control equipment was used? At first, linear amplifiers with power transistors and outputvoltages up to about 100 V. Later, thyristor converters were used as well and towards the end of theseventies, DC chopper converters based on switching transistors came to the fore.This compensated to a great extent for the initial low level of efficiency of the electric actuators. Thevoltage that could be output by the DC motors was nevertheless still only about 200 V as a result ofthe insufficient blocking voltage of the transistors and the limited voltage between the segments ofthe commutators.DC chopper converters had to be connected to the mains via an isolating transformer. At the sametime, this transformer isolated the actuator from the mains.Control of both speed and torque was analogue, with all the associated problems of susceptibility oflow signal voltages to interference across the wide speed range typical of servo drives. A DCtachometer was used as feedback unit to measure actual speed.The development of frequency inverters, initially based on thyristors, later on power transistors, ledto the increasing use of low-wear AC squirrel-cage motors and standard motors in the case of drivesnot requiring such precise control.

Drive Engineering - Practical Implementation - Volume 7

IntroductionThe search for brushless motors that could be used in servo drives has been underway since themid-seventies.A reversal of the principle of the conventional DC motor appeared to be a promising solution: armaturein the stators, field-excitation in the rotors. The brushless DC motor or electronic commutator motorwas born.In principle, this motor is a permanent-field synchronous motor that requires a simple positionencoder issuing 6 signals per revolution to determine the position of the rotor.In addition to electronic commutation and the consequent low rate of wear, this type of drive has thefollowing advantages:

lower moment of inertia through the use of unwound rotors

simpler cooling, as power loss occurs in the stators rather than the rotors

greater efficiency, as there is no loss caused by the excitation winding

Electronic commutation of the current produced by the stator winding occurs every 60 elec. and iscontrolled by the position encoder. As DC blocks are commutated, this principle is also called blockcommutation. An additional encoder is required to control speed.In parallel with these developments, further work has also been carried out to develop the ACsquirrel-cage motor as a brushless servo drive. This type of motor is cheap to manufacture and hasthe additional advantage of being able to operate in the field-weakening range.Another way towards the brushless drive was the development of what is known as the sinusoidallycommutated servo drive:The principle behind this motor is also the permanent-field synchronous motor, with its advantagesas mentioned above. The position encoder for the rotor in this case, however, is a resolver, the outputsignal from which controls the sinusoidal stator current.Examples of all three of these principles for brushless servo drives are in use today and have almostcompletely replaced drives with brushes since the start of the nineties.The decisive factor in their success has been the progress that has been made in the field ofsemiconductors. The development of highly integrated, high-speed processor systems andnon-volatile memory modules has facilitated the introduction of digital control. Whether or not somefunctions are required more often or less often in individual systems is no longer significant as faras cost is concerned. Individual software instead of a multiplicity of bits of hardware can be used toimplement everything.The power section in the controller for all three types of brushless systems is basically the same: afrequency inverter that is controlled by the machine rather than the self-controlled inverters used forstandard motors. The only functional differences are in the areas of open-loop and closed-loopcontrol.Developments in power transistors since the early nineties have also made it possible to connect theservo controller directly to the mains voltage without having to use a mains transformer.

Drive Engineering - Practical Implementation - Volume 7

Introduction

1.2.2 Development of the market for servo drives

If the servo drive was initially only to be found in machine tool applications, its potential was veryquickly realized in the early seventies as a result of automation in the growth areas of materialshandling, industrial robots and automated assembly. In contrast to machine tool applications, thereplacement of pneumatic and hydraulic equipment was slower owing to the often very differentrequirements placed on the drives.Because of their low mass, the materials handling and robotics industry initially most often employedDC pancake motors, frequently in conjunction with low-backlash planetary gear units or othercompact special-purpose gears. The pancake motors were themselves later replaced by brushlessmotors.Nowadays, automation is in full swing in all areas of the mechanical engineering sector, the electricaldrive is dominant and the mechanics of the machines have been greatly simplified by using modernsingle drives instead of central drives. As a result, the market for servo drives has expanded. Today,there is hardly any area of engineering in which there are no applications for servo drives. The mostimportant are:

paper processing

sheet metal processing

packaging

materials handling systems

wood working

building materials processing

As servo drives are used very differently in all sectors, not all the applications are highly dynamic.The features of high steady-state or dynamic control precision, the wide speed range, the high surgewithstand capability or even just the low weight or small size are often by themselves the decisivepurchasing factors.Thanks to modern digital technology, servo drives are much easier to use than they were a coupleof years ago. Digital technology provides a wide range of application-related options, interfaces toall controls (either directly or via a bus system) and the ability to use a PC to commission, optimizeand automatically calibrate the drive.The hydraulic and pneumatic solutions mentioned at the beginning of this section are now confinedto niches in the market.

Drive Engineering - Practical Implementation - Volume 7

Introduction1.3 A comparison of the most common drive systemsIf a comparison is to be made of drive systems common with and available from SEW, differentfactors must be used as a basis for comparison. Comparative criteria must be chosen with great careto avoid comparing apples and oranges.We shall take a closer look at three principal areas:

motor characteristics

basic drive characteristics

system configuration in a given application

1.3.1 Comparing motor characteristics

The first comparison to be made is that of the motors. We shall compare motors of the same speedand power rating.CharacteristicsPower [kW]Speed [1/min]

AC asynchronousmotor (direct-on-line)

DC motor

Permanent-fieldsynchronous motor

7.5

8.3

7.5

2900

3200

3000

Type / Size

DFV 132 M2

GFVN 160 M

DFY 112 ML

Enclosure

IP 54

IP 44

IP 65

fan-cooled

fan-cooled

self-cooled via surface

Length [mm]

400

625

390

Total weight [kg]

66

105

38.6

Weight of rotor [kg]

17

29

8.2

280

496

87.4

24.7

24

1.6 MN

3 MN

Cooling

Jmot

[10-4

kgm]

Rated torque [Nm]

24.72.6 MN / 1.8

Maximum torque Mmax

Max. angular acceleration

[1/s]2)

MN1)

1588

797

8238

Max. dynamic performance [%]4)

(servo motor = 100%)

20

10

100

Acceleration time tH3) [ms]

191

420

38

1) Given are the pull-out torque and the mean acceleration torque MH, which is used for calculation.2) Max. angular acceleration =

Jmot

Jmot nmot

3) tH =

4)

Mmax

9.55 Mmax

servo

100%

Some typical motor characteristics are evident just by looking at the table. Shaded motorcharacteristics shall be dealt with in greater detail.

Drive Engineering - Practical Implementation - Volume 7

Introduction

Weight of motors and rotors

12010080Weight [kg]

6040

10566

200

38.6

17

29

DFV 132 M2

GFVN 160 M

8.2DFY 112 MLMD0100AE

Fig. 1: Weight of motors and rotors

Figure 1 shows the weight of the different motors in comparison. It is obvious that the synchronousmotor has by far the lowest weight. In particular in systems where the drive is traveling together withthe system a low motor weight is of major advantage.The power/weight ratio of the motors compares as follows:

asynchronous motor:

DC motor:

synchronous motor:

8.8 kg/kW12.7 kg/kW5.2 kg/kW

Motor moment of inertia

5004002

J [kgm ]

300

4962001000

28087.4DFV 132 M2

GFVN 160 M

DFY 112 MLMD0101AE

Fig. 2: Motor moment of inertia

Figure 2 compares the mass moment of inertia of the motors. Again, the difference between the servomotor and the DC motor in particular is striking. A small mass moment of inertia of the motor is ofadvantage particularly in terms of dynamic performance. It is unfavourable however when largerexternal masses are to be moved.

Drive Engineering - Practical Implementation - Volume 7

IntroductionDynamic performance100%80%60%

100%40%20%

20%0%

10%

DFV 132 M2

GFVN 160 M

DFY 112 MLMD0102AE

Fig. 3: Dynamic performance

Figure 3 clearly shows the lead the synchronous motor has over the other systems in terms ofdynamic performance.Acceleration time without load

tA [ms]

450400350300250200150100500

42019038DFV 132 M2

GFVN 160 M

DFY 112 MLMD0103AE

Fig. 4: Acceleration time without load

With its maximum motor torque Mmax and its low mass moment of inertia Jmot the synchronousmotor has a very short acceleration time when not loaded which highly recommends it for dynamicapplications.

10

Drive Engineering - Practical Implementation - Volume 7

Introduction

1.3.2 A comparison of basic drive characteristics

Controlled drives have special characteristics which influence drive selection.Characteristics

Positioning performance and

clearly not as good

as that of theasynchronousmotor withclosed-loop speedcontrol (dependson peripheralconditions (PLC,brake, etc.))

positioning accuracy toapprox. 50 angularminutes

positioningaccuracy toapprox. 5angular minutes

1) When the motor is fed from the frequency inverter with a speed control option, appropriate selection of the frequencyinverter will provide a motor torque of 300% MN and more.2) Third-party motors in current-controlled systems require specific knowledge of the motor characteristics to properlyadjust the current controller. Furthermore knowledge is required of the technical details of the feedback and theevaluation systems, the commutation method, etc. Therefore, it is common practice, in particular in servo technologyto purchase all components from one manufacturer (one-stop-solution).3) Taking into account the maximum permissible braking work, SEW DFY brake motors can handle several emergencystop braking operations.4) In the synchronous motor the mechanical brake is a mere emergency and holding brake.

Drive Engineering - Practical Implementation - Volume 7

11

Introduction1.3.3 A comparison of different system configurations in a given applicationHaving concluded all initial observations, the next step is to configure different systems and tocompare the systems performance in a given application. All systems compared were chosen withthe same power rating and output speed as a basis for comparison.Load data: m = 1000 kg; vmax = 1.5 m/sCharacteristics

1) Not included are the response times of PLC, frequency inverter and contactor, the brake release reaction times and thegear unit backlash.Brake reaction times for switch-off in the AC and DC circuits are considered.

1.4

Advantages and disadvantages of a servo drive

Advantages:

12

excellent speed holding

excellent dynamic performance

wide speed range

high positioning accuracy

static torque (zero speed)

high overload capability (3 Mo)

Disadvantages:

relatively high system cost

Drive Engineering - Practical Implementation - Volume 7

Introduction1.5

Components of a servo system

79

SEW

EURODRIVE

SEW

EURODRIVE

3O N24VT R IPStat e/Adr.1

S16

X 01

X311

X21 X32

12

X 0 2X 0

X0

10

8MD0079AX

Fig. 5: Components of a dynamic drive

Figure 5 shows the components of a servo system. Essentially, the following components arerequired:12345

Motor with/without gear unit

The following sections will deal with these components in greater detail.1.6 Overview of common servo motorsUntil a few years ago, servo drives were implemented in DC technology with brushlesspermanent-field motors with either thyristor or transistor power controllers.Brushless AC permanent-field synchronous motors, as produced, among others, by SEW areincreasingly used nowadays. Their advantages over DC drives are:

better price/performance ratio

better performance/weight ratio

longer service life

high thermal load rating

Drive Engineering - Practical Implementation - Volume 7

13

IntroductionServo motors can be divided into several groups:Servo motor

Brushed

DC motors

Brushless

DC motors

AC motors

Permanent-fieldDC motors

Permanent-fieldAC synchronousmotors

Stepper motors

Asynchronousmotors with fieldoriented controlMD0080AE

Fig. 6: Classification of servo motors

The distinguishing features are in the design of the motor, in the controller design and the type offeedback system used.The three major systems are briefly explained here:

asynchronous motor with squirrel-cage rotor and field-oriented control (Sec. 1.6.1)

1.6.1 Asynchronous motor with squirrel-cage rotor and field-oriented control

The asynchronous motor with squirrel-cage rotor and field-oriented control is also termed an ACservo motor. In its basic structure and mode of operation this motor corresponds to the well-knownthree-phase asynchronous motor with squirrel-cage rotor.As servo motors, asynchronous motors are designed with low-inertia, low-leakage and low-slip rotorsand are operated with a special control which ensures that stator and rotor flux are alwaysperpendicular to one another. This allows the asynchronous motor to be operated at almostbreakdown torque whenever dynamic responses are required, making it highly suitable forhigh-dynamic applications.The disadvantage of this motor (compared with the permanent-field machine) is its lower efficiencyand somewhat greater unit volume in relation to the torque. Current-dependent losses occur in therotor which do not occur with a permanent-field rotor. Because of the higher losses (efficiency )and the magnetizing requirements (power factor cos ) of the asynchronous machine, it requires aninverter output higher by the inverse ratio of cos 1 cos Further heat dissipation measures have to be taken especially in the range of lower speeds. Thesemotors are then usually provided with forced cooling fans or the speed control range or torque isreduced.

14

Drive Engineering - Practical Implementation - Volume 7

The costs compared with other systems are higher due to the complexity of the signal processinginvolved in high-dynamic applications. It is mainly the high-resolution encoder and the fast andefficient microprocessors which are responsible for that. The processor must continually calculatethe stator currents from the rotor position and the required torque-producing and magnetizingcomponents.Previously these drives were generally used as high-output main drives in the machine tool industry.However, the use of these drives can be expected to increase because electronic components arebecoming cheaper and the motors can be produced more economically.The torque-speed characteristic shows the curve withM

continuous torque

maximum torque

characteristic of thestandard asynchronous motorin comparison

Mmax

23

MN

1n0

nMD0081AE

Fig. 7: Torque-speed characteristic

Characteristic 2 shows the envelope which depends on the DC link voltage of the inverter and itscurrent capacity.

1.6.2 Permanent-field synchronous motor

The permanent-field synchronous motor, sometimes also referred to as electronically-commutatedmotor or brushless DC motor is currently the motor which best satisfies the requirements made ofa servo system. The stator can be compared directly with that of the asynchronous motor. Thelaminated rotor has adhesively attached magnets which provide the constant magnetic field. Themotors are normally enclosed (minimum IP 54) and fan-cooled.The motor can be operated with different current injection methods, block-type and sinusoidalcommutation techniques having been accepted in this case. The difference lies in the induced currentflow and in the type and implementation of the feedback systems.

Drive Engineering - Practical Implementation - Volume 7

15

Introduction1.6.2.1 Permanent-field synchronous motor (with block-type commutation) /Brushless DCThe AC permanent-field synchronous motor with block-type commutation as described in thefollowing is often called the brushless DC motor.In block-type commutation, the current controller and power output stages are controlled by a rotorposition encoder (RLG). These could consist of Hall sensors, photoelectric sensors, or somethingthe like.A major advantage of the block-type commutation is simple generation of position signals and theirconversion to control signals for the current.The curves of the individual characteristic parameters are shown in the figures below.Current, voltage and flux over timeat constant speediU

t1 t2

360el

iVel

Equivalent circuitiU u 1iV v 1iW w1

VL UVL VVL W

Vind U

u2

iW

el

Vind V

v2

~Vind W

w2

Vind U

Vind: Voltage induced by rotation of the rotor

el

Vind Vel

VL : Voltage drop due to inductivity

Vind W

el

Rotor position at the time

t1

U1

t2

W2

U1

W2

NV1

V2

V2

SU2

W1

el

V1BV

SW1

BU

el

U2BW

el

MD0089BE

Fig. 8: Block-type commutation

16

Drive Engineering - Practical Implementation - Volume 7

Introduction

Block-type currents are injected into the motor windings as a result of which trapezoidal voltages areinduced in the motor. The design then produces rectangular distribution of the air-gap flux density.This results in constant torque development.Two adjacent phases are always fed with current in block commutation.The rotor position encoder is used to detect the rotor position, and brushless tachogenerators todetect the speed.

Position

Speed

Current

Gearunit

MRLG

Load

Rotor position encoder

Tachogenerator

Encoder

MD0088AE

Fig. 9: Control structure of the block-commutated motor with encoder

Figure 9 shows the components of a control loop in the block-commutated motor. It shows clearlythat a dedicated actual-value encoder plus wiring is required for each controlled variable.

Vind: Voltage induced by rotation of the rotor

el

Vind Vel

VL : Voltage drop due to inductivity

Vind W

el

Rotor position at the time

t1

t2

U1

U1

W2

W2

V1

el

V1

V2

V2W1

U2

W1

el

U2

WelMD0106BE

Fig. 10: Sinusoidal commutation

Basically, the commutation sequence takes place on the same principle as in block-type commutation.The differences are that all the three phases are now simultaneously fed with current, and that thecurrent, the induced voltage and the flux are sinusoidal. This means that torque and speed stabilityare achieved, even at low speeds. Additional measures in the mechanical design of the motor aid this.

18

Drive Engineering - Practical Implementation - Volume 7

Introduction

The sinusoidal-fed motors are normally equipped with resolvers as feedback systems. Resolvers arecertainly more complex in their evaluation, however offer higher resolution because the evaluationis digital, and save one feedback system, in particular when there is a position control superimposed,thereby reducing the wiring requirements as well.

Position

MM

Current

Speed

R/D converter

Gearunit

Load

Resolver

MD0090AE

Fig. 11: Control structure of a sinusoidally commutated motor

Motor control of a sinusoidal-fed permanent-field synchronous motor is described in detail in Sec. 4.

1.7

Feedback systems

An feedback system is used to detect specific drive data, including:

speed

load angle (position within one revolution)

machine position (position over several revolutions)

1.7.1 Overview of the most common feedback systems

Feedback systems

Digital feedback systems

Analogue feedback system

Tachogenerators

Resolvers

MD0091AX

Incremental encoders

A, A, B, B, C, C

Absolute encoders

Gray code

Dual code

MD0091AE

Fig. 12: Overview of feedback systems

Drive Engineering - Practical Implementation - Volume 7

19

IntroductionThe different feedback systems supply the following data:Feedback system

Supplied dataRotor phase angle

Machine position

Speed

Absolute encoder singleturn

(X)

(X)

Absolute encoder multiturn

(X)

(X)

(X)

(X)

(X)

Incremental encoderResolver with R/D converterTachogenerator

X can be evaluated directly, (X) available with additional evaluation

An important criterion when selecting a feedback system is its ruggedness. Since the feedback systemis mounted directly on the motor, it must be able to handle higher temperatures and vibration. Anotherimportant factor is the feedback systems RF noise immunity.1.7.2 Advantages and disadvantages of the major feedback systemsFeedback systemIncremental encoder

Advantage relatively rugged designs available

large selection of resolutions,

mounting types, interfacesposition data is still available after apower failureclear assignment of position andoutput valuevery high resolution available

rugged design

insensitive to vibrations and higher

temperaturelow wiring requirement

can be fitted in motor

economizes on additional feedback

systems

Absolute encoder

Resolver

Disadvantage position data is lost in the case of apower failure

high cost

more complex evaluation

When weighing the pros and cons of the different feedback systems the resolver recommends itselfas highly suitable for use with the servo motor.

20

Drive Engineering - Practical Implementation - Volume 7

The permanent-fieldsynchronous motor2

The permanent-field synchronous motor

2.1

Design and method of operation

1

00037AXX

Fig. 13: Sectional view of a permanent-field synchronous motor

Explanation:

1. Stator housing2. Stator winding3. Laminated stator core

4. Rotor with permanent magnets

5. Resolver6. Brake

Synchronous motors are polyphase machines in which the stator rotating field and the rotor rotatingfield run synchronously.A rotating field is generated by the spatial arrangement of the stator coils and the chronological phasesequence of the input current.The speed of the rotating field nd is derived as follows:

nd =

f 60p

where: f = frequency of the applied voltage

p = stator pole pair number

SEW synchronous motors are always 6-pole motors (p = 3).

f [Hz]nd

[min-1]

100

150

225

2000

3000

4500

The speed as a function of the frequency when p = 3.

SEW permanent-field synchronous motors are designed as 6-pole motors since the use of 6-polemotors makes for minimal iron losses at 3000 min-1 (150Hz) and at the same time ensures goodtorque stability with low magnet requirement.

Drive Engineering - Practical Implementation - Volume 7

21

The permanent-fieldsynchronous motorSEW synchronous motors are, fundamentally, star-connected.As with the asynchronous motor, the stator consists of the housing, the laminated core assemblyand the stator winding. The rotor consists of a shaft, rotor laminates and adhesively attachedpermanent magnets. To improve the dynamics of the motor the laminates of the rotor are not solidbut with cutouts (see Fig. 14). This reduces the rotors moment of inertia and, thus, its accelerationtime.The permanent magnets used are of the rare-earth material neodymium-iron-boron. Magnets in thismaterial have particularly good magnetic properties compared with the previously used ferritemagnets and can produce higher torques.

123

00038AXX

Fig. 14: Sectional view of stator and rotor

Explanation:

1. Stator core assembly

2. Winding slots3. Rotor laminates4. Permanent magnet5. Cutouts

2.1.1 Function of the AC permanent-field synchronous motor

If the motor is connected to a suitable controller, a rotating field - the so-called stator rotating field- is produced in the windings of the stator. This rotating field operates on the rotor and exerts a forceon it. Because of the magnetic coupling between stator and rotor, the rotor is accelerated into thisfield and runs at the same angular velocity, i.e. synchronously.If the motor is loaded, a lag of the rotor rotating field in relation to the stator rotating field is produced.The poles of the rotor lag to those of the stator rotating field by a certain angle, the load angle . Thegreater the load angle, the more the torque increases. If the load angle is precisely 90, i.e. the poles

22

Drive Engineering - Practical Implementation - Volume 7

The permanent-fieldsynchronous motor

of the rotor lie precisely between two stator poles, then the force operating on the rotor is at itsmaximum. The stator pole leading the rotor pole pulls the rotor and the lagging stator pole pushesit, producing the described effect. If the load angle is further increased, i.e. the motor is overloaded,the torque decreases again, motor operation becomes unstable, the motor stalls and comes to astandstill.Where: M = f (V, I, sin )M

M = f(sin)

-180 -90

+90

+180

MD0092AX

Fig. 15: The torque as a function of the load angle

2.1.2

Motor control

To be able to operate a synchronous motor with maximum possible torque, it must be ensured thatthe load angle is 90. This means that the stator field must always lead by 90 when the drive ismotoring and lag by 90 when it is regenerating. The motor control calculates the three phase currentsof the motor from a given torque and reads out the current setpoints from a table.For this purpose the rotor position is sensed with the position encoder. 90 is added or subtractedto or from the value of the position angle, according to direction of rotation and direction of torque,and the associated currents are then calculated.The appropriate position of the stator rotating field is determined for each rotor position, i.e. the rotordetermines the magnitude and direction of the stator field. Thus the rotor rotates the stator field.The load angle mentioned in this context is always the electrical angle. In a six-pole motor 90electrical corresponds to 30 mechanical.

Drive Engineering - Practical Implementation - Volume 7

23

The permanent-fieldsynchronous motor2.1.3 Current relationships in the statoriU

1800

iUiV

15360

220

90

iV

iW

iW

Current vector I = vectorial

sum of currents i U, iV, iWI

With respect to torque generation

the relationships in the stator atdifferent moments in time areto be imagined as:

Fig. 17: Generation of current instantaneous values

24

Drive Engineering - Practical Implementation - Volume 7

The permanent-fieldsynchronous motor2.2

Speed-torque characteristic

Three limits can be seen in the speed-torque characteristic of a DFY motor. These must be consideredwhen configuring a drive.1) The maximum torque of a motor is limited, among other factors, by the load rating of the permanentmagnets. If a motor is too heavily loaded and the current increases to an excessive value, the magnetsbecome demagnetized and the motor loses its torque.No demagnetization can occur with correct selection and matching of motor and controller.2) Further attention should be paid to the limitations in torque in the upper speed curve, which arecaused by the voltage. Voltage here refers to the voltage which is present on the motor terminals.It depends on the DC link voltage, the mains supply voltage, and the voltage drop in the cables. Thereason for the decrease in torque is that the maximum current can no longer be injected into themotor due to the back e.m.f. (voltage induced in the motor). The motor can no longer attain themaximum torque.3) A further limitation is the thermal load of the motor, which must additionally be calculated in thedesign. Rms torque is calculated in this case, and it must be smaller than the static torque M0. If thethermal limit rating is exceeded, the magnets will become demagnetized.

Drive Engineering - Practical Implementation - Volume 7

25

The permanent-fieldsynchronous motor2.3

Electromechanic emergency and holding brake

The brake which is used in the larger frame-size servo motors is based, in its mechanical layout, onthe service brake of the asynchronous motor. However, in servo technology this brake is only requiredas emergency and holding brake, as braking and holding are performed electrically. Even though thebrake used in SEW synchronous motors is only employed as emergency and holding brake, it canstill produce a high holding torque (3M0) and do considerable braking work. This makes synchronousbrake motors by SEW particularly suitable for hoisting applications.The brake is normally only used in the event of:

longer stationary periods (reducing the thermal load on the motor)

emergency stops

The optional disc brake is fully integrated in the motor. Removal and installation can be performedon site, without interfering with the motor.The brake has a separate plug connector for its independent electrical supply. Standard brake supplyvoltages are 230 VAC, 400 VAC and 24 VDC.1

Working air gap

00039AEN

Fig.19: Sectional view of the brake

26

Drive Engineering - Practical Implementation - Volume 7

The permanent-fieldsynchronous motor

The brake is a DC excited electromagnetic disc brake which is released (opened) electrically, i.e. whenpower is supplied, and brakes by spring force. The system satisfies basic safety requirements. Thebrake is applied automatically if the voltage is removed or in case of a voltage failure.The disc brake operates on the well-proven two-coil principle. The brake rectifier or brake controlunits initially only energize the accelerator coil. As soon as the brake is released, the system switchesover to the holding coil electronically.Minimum wear together with maximum service life and high switching capacity are the outstandingfeatures of this brake system.The I2R losses are reduced as much as possible in continually-released operation, so that thermalload on the brake is very small.The braking torque is determined by the type and number of brake springs. Brakes with higher brakingtorque (up to 3 Mo) are preferred for hoisting applications.Brake rectifiers and brake control units are used for brake control. The brake rectifier is used in thecase of AC connection and the brake control unit in the case of 24 VDC connection. For reasons ofspace, both are installed in the switch cabinet and not in the terminal box.The brake rectifier is designed as a one-way rectifier with protective circuits against overvoltages,and integral control electronics to shorten the response times of the brake.Brake response timesBrake motor frame sizeBraking torque [Nm]

56B

71B

90B

112B

2.5

10

15

12

20

30

40

17.5

35

60

90

Brake releaseBrake release reaction timet1 [ms]

10

12

16

20

13

15

18

22

11

14

22

35

Brake applicationBrake reaction time t2 [ms]

95

45

20

28

20

13

10

130

60

32

20

Drive Engineering - Practical Implementation - Volume 7

27

The permanent-fieldsynchronous motorFor 24 VDC supply the BSG brake control unit is available. It corresponds in function to the BME brakerectifier with the difference that because brake control is with direct current, the brake control unitdoes not have a brake rectifier.VAC

1234131415

PE

MAS ... /MKS ...BME

Switch cabinet

DFY

BrakeConnector

BrakeMD0107AE

Fig. 20: Brake control

Figure 20 shows a brake rectifier for the switch cabinet. The brake rectifier is wired to disconnectboth the DC and the AC circuits, i.e. fast application of the brake. Rapid excitation is also obtainedwith this type.You will find more detailed information about the brake in Drive Engineering - PracticalImplementation, Volume 4.

28

Drive Engineering - Practical Implementation - Volume 7

The resolver

The resolver

3.1

Design and function of the resolver

The resolver operates on the principle of a rotary transformer. In a rotary transformer the rotorconsists of a coil (winding) which together with the stator winding forms a transformer.The resolver is basically designed exactly this way, with the difference that the stator is made up oftwo windings displaced by 90 to one another, instead of one winding.The resolver is used to determine the absolute position of the motor shaft over one revolution.Furthermore, the speed and the encoder simulation for the position control are derived from theresolver signal.

V1

V2

VR

MD0116AX

Fig. 21: Resolver

MD0108AE

Fig. 22: Schematic design

The rotor of the resolver is mounted on the motor shaft. Both the stator and the rotor are providedwith an additional winding each to allow for brushless transmission of the stator primary voltage tothe rotor. With the aid of these additional windings the primary voltage of the stator winding istransmitted on the transformer principle. The two windings carried on the rotor are coupledelectrically so that the voltage transmitted from the stator to the rotor is also present on the secondwinding of the rotor.

Drive Engineering - Practical Implementation - Volume 7

29

The resolver

stationary

stationary

rotating

R1

S2rotor

statorVe

statorV2

VR

R2

S4statorV1S1

stationaryS3MD0109AE

Fig.23: Resolver block diagram

Voltages of different magnitude are induced in the stator windings, depending on the position of therotor. The winding through which there is full current flow at = 0 (see Fig. 22) has the maximumvoltage present at this point in time. If the rotor is rotated, then voltage V1 on this winding decreasesuntil it has attained the value zero at an angle of 90. If the rotor is further rotated, the voltage againincreases with inverse polarity until it has again reached its maximum at 180. Voltage V1 has a cosinecurve as envelope. Voltage V2, which is displaced by 90 to voltage V1, has a value of 0 V at 0. Itincreases until it has attained its maximum value at 90, and then decreases again. The envelope ofV2 is therefore a sine curve.The output voltages U1 and U2 are calculated as a function of the input voltage Ve as:Input:Ve = VS sint (reference voltage)Output: V1 = VS sint cosV2 = VS sint sinwhere

= rotor angle = angular frequency of VeVS = input voltage peak value

V1

V200058AXX

Fig.24: Output voltages V1 and V2 of the resolver

30

Drive Engineering - Practical Implementation - Volume 7

The resolver

3.2 Processing and evaluating the resolver signals

The signals of the resolver are converted in the R/D converter (resolver/digital converter) into a digitalnumerical value. This digital value can be further processed to obtain additional information. The R/Dconverter provides information on the rotor position. Using the count value, the speed of the motorcan be determined by counting the number of pulses within a specific time window, which then servesto determine the speed. The two least significant bits of the of the count value can be evaluated:- for encoder simulation to determine the speed- for higher-level positioning controls.Ve(Reference)Resolver

ReferencesignalSignalprocessing

V1 (cos)Evaluation

Rotorposition

Speed

12 Bit

V2 (sin)Monitoring

Pulses

MD0112AE

Fig. 25: Processing of resolver signals

The oscillator [1] (see Fig. 26) feeds the rotor via the stator winding with an AC voltage of about 10 Vrmsand a frequency of about 7 kHz. The digital numerical value of the up-down counter [6] is thenconverted in a D/A converter [5]. The output signals V1 and V2 of the stator of the resolver aremultiplied by the sine or cosine of the converted value. The value of the up-down counter representsthe angle . As a result, the two voltages below are produced:VF1 = VS sint sin cosVF2 = VS sint cos sinThe two multiplied output signals are subtracted from one another in the error amplifier [2]. Thedifference corresponds to the error (deviation) between the angle and the actual angle . The erroris:VFD = VS sint (sin cos - cos sin)Simplified, this equation is:VFD = VS sint sin( - )This signal is demodulated in the phase-selective rectifier [3] - which is downstream of the subtractor[2] - in order to remove the carrier frequency. The signal arising at the output of the rectifier is theerror voltage VF, which is proportional to sin ( - ).This voltage is applied simultaneously to an output of the R/D converter and the input of the integrator[4]. The integrator [4] integrates the error voltage which is applied to the input of a voltage-controlledoscillator (VCO) [7].

Drive Engineering - Practical Implementation - Volume 7

31

The resolverIf there is an angular difference between the angles and , the integrator produces a DC voltagefrom it. Using this DC voltage, the VCO [7] produces pulses, which are then processed in the up-downcounter [6].Oscillator1

Stator

Ve = VS sintV1 = VS sint cosV2 = VS sint sin

CosinemultiplierSinemultiplier

VF

VF1VFD

32

VF2

D/A5converter

Synchronousrectifier

Integrator

VCO

Up-down6counterResolver

VT

POSITION(12 Bit)

Evaluation and signal processing

MD0113AE

Fig. 26: Block circuit diagram of an R/D converter

The modules [2] to [7] form a closed-loop control circuit. A DC voltage signal is present at the VCO[7] until the difference between the angles and equals zero, i.e. until:=Thus the digital value of the up-down counter corresponds to the analogue value of the angle of theresolver present at the input of the R/D converter.Over a continuous turn of the resolver the VCO must produce pulses until the count value of the V/Rcounter corresponds to the analogue value of the rotor angle at the input, i.e. the angular variationof the resolver is offset. Consequently, the frequency of the VCO is proportional to the speed of themotor and the resolver. From this it follows that the output voltage of the integrator is also proportionalto the speed.The R/D converter supplies a direct voltage VT at the outputs, which is proportional to the speed,plus absolute information for one revolution of the resolver.The evaluation circuit is implemented as an integrated circuit, only the oscillator [1] is connectedexternally.The error of the resolver signal is negligible (< 0.05%).

32

Drive Engineering - Practical Implementation - Volume 7

The resolver

3.3Encoder simulationThe encoder simulation produces a total of six tracks from the already available output signals of theresolver. These are used by higher-level controls for positioning. The six tracks are A, B and C andtheir negations A, B and C.The encoder simulation provides 1024 pulses per revolution. Through quadruple evaluation 4096pulses per revolution are available for positioning controls.AA18036090BB90CC

MD0114AE

Fig. 27: Encoder simulation

The pulses of tracks A and B are displaced by 90. If the positive edges of the pulses of track A leadthose of track B, then the motor is rotating clockwise. If track B leads track A by 90, the motor isrunning counterclockwise.For each full revolution of the motor, i.e. when going through zero position, track C supplies a pulse:

20

Counterclockwise

21

Clockwise

The motors direction of rotation can be determined with the aid of the two least significant bits (LSB)of the signal processing.

Function table of the two LSBs

For clockwise (positive) rotation the counter in the R/D converter counts up. The function table isthen read from top to bottom. Each time the least significant bit 20 changes from 1 to 0, the value ofbit 21 changes too.If the motors direction of rotation changes, the function table must be read from bottom to topcorrespondingly. In this case, however, when the least significant bit 20 changes from 1 to 0, thevalue of bit 21 does not change.

Drive Engineering - Practical Implementation - Volume 7

33

The servo controller

The servo controller

The servo controller is used for speed and torque control of the servo motor. Nowadays, this isnormally a digital controller. The digital controller has the following advantages over the analoguecontroller:

more resistant to ageing

drift-free

simple communication

computer operations easily implemented

Servo controllers are used both in the form of compact servo controllers (so called stand-alone units)as well as in modular designs.

MD0061AX

Fig. 28: Servo controllers in modular design

MD0104AX

Fig. 29: Compact servo controller

Stand-alone units have the advantage that the servo controller is available as a complete unit. At thesame time, the additional wiring between the individual unit components - as is necessary in themodular system - is eliminated.The advantages of the modular digital servo controller (power supply module + axis module) are inmulti-axis applications. Several axis modules can be supplied by one common power supply modulein multi-axis applications. The required output capacity of the power supply module is determinedfrom the total power requirements of the connected axis modules and their utilization.A digital servo controller of modular design is described in the following sections.

Fig. 30: Structure diagram of a modular servo controller

The modular servo controller consists of two basic components

power supply module

axis module

The power supply module is used for the power supply to the connected axis modules via the DClink, and for the voltage supply to the control electronics. It also contains the central brake chopperor the mains energy feedback unit, various protective features and standardized communicationsinterfaces (RS-232 and RS-485).The axis module controls speed and torque of the servo motor. It contains the control electronicsneeded for this, permanently assigned and freely programmable binary inputs and binary outputs,analogue inputs, analogue outputs, the output for the encoder simulation and a free slot for the optionpcbs.The number of axes which can be connected to a power supply module is limited by the:

output capacity of the power supply module

output of the switch-mode power supply

maximum braking power

line length of the DC link, the data line bus connection (interference immunity) and the 24V busconnections.

Power supply module

L1L2L3

Axis module

M3~

MD0155AE

Fig. 31: Power section of a servo controller

Drive Engineering - Practical Implementation - Volume 7

35

The servo controller

The power section of the servo controller is based on the principle of the static voltage DC linkconverter. This means that capacitors keep the voltage stable on the DC link. The output stage orpower inverter transistors are IGBT transistors. Their advantages are low switching losses, simplecontrol, low forward power losses and high switching frequencies.4.2

Power supply module

The power supply module is connected to the three-phase mains supply through a line choke on thesupply side. The supply voltage range is 3 380 ... 500 V. The line choke, in conjunction with designmeasures in the power section of the controller, completely replaces other customary inrushcurrent-limiting charging components. It minimizes noise on the supply lines and is part of the unitprotection features against transient overvoltages.

L1L2L3Linechoke

M3~MotorOvervoltageprotection

Rectifier

RS-232

DC link

Brakechopper

Resolver

Inverter

EvaluationMonitoringsection

RS-485

24 V ext.

ControlOptions

Switch-modepower supplyVDC link24 V

Power supply module

Switch-modepower supply24/15/+5V24VAxis moduleMD0156AE

Fig. 32 : Block circuit diagram of a modular servo controller

The power supply module includes the following monitoring features:

36

DC link overvoltage

mains phase failure

earth fault

overtemperature

brake chopper overcurrent protection

Drive Engineering - Practical Implementation - Volume 7

The servo controller

4.2.1 Rectifier and overvoltage protection

Inside the power supply module a surge suppressor circuit protects the power section againstdamage that may be caused by voltage peaks in the supply lines, which occur when inductive andcapacitive loads are connected to the mains.The overvoltage protection is implemented by means of capacitors, surge arresters and varistors.+VDC linkF1L1F3R1L2F2R3

L3R2C1

C2

C3

-VDC link

C4

MD0158AE

Fig. 33: Rectifier and overvoltage protection

The input rectifier is a three-phase bridge rectifier.

4.2.2 DC link and mains energy feedback

When a drive is decelerating kinetic energy is converted into electrical energy and this is fed backinto the DC link. As the capacity of the DC link capacitor is limited, the voltage in the DC link rises.To enable the drive to decelerate, this additional energy must be dissipated.It is therefore necessary to store the regenerated energy or to convert it into other forms of energy.There are basically three possibilities for this:

energy feedback to the mains (the electrical energy can be used by other consumers)

brake chopper and braking resistor (the energy is converted into heat)

exchange of energy in multi-axis applications (the electrical energy is used by other connectedmotors).

Drive Engineering - Practical Implementation - Volume 7

37

The servo controller

4.2.2.1 Supply energy feedbackThe advantage of the mains energy feedback is that the energy is fed back into the supply networkand therefore remains available as electrical energy.There are various options for implementing mains energy feedback, one is the anti-parallel bridge.With this form of mains energy feedback, the mains rectifier is expanded by a bridge circuit of sixpower transistors which are triggered synchronously with the mains. If the DC link voltage exceedsthe rectifier value, then regenerated energy is fed back into the supply.Power supply moduleAxis module

L1L2L3

M3~

Rectifier

Inverter

InverterMD0159AE

Fig. 34: Mains energy feedback

4.2.2.2 Brake chopper and braking resistor

In contrast to mains energy feedback, the produced energy is not fed back into the supply butconverted into heat by the braking resistor. If only little braking energy is produced it may be lessexpensive to use a brake chopper rather than the mains energy feedback.

BW

VDC linkControlssignals

VDC link

VDC link

MD0160AE

Fig. 35: Brake chopper

38

VDC linkMD0161AE

Fig. 36: Switching behaviour of the brake chopper

Drive Engineering - Practical Implementation - Volume 7

The servo controller

4.2.2.3 Comparison between mains energy feedback and brake chopper

A decision must be made as to which method is most suitable for a given application and its specifics.Mains energy feedback

Brake chopper and braking resistor

Location

Completely integrated in the power

supply module

Brake chopper in the power supply

module, braking resistor externally or inthe switch cabinet

Effect on the ambient temperature

Minor

Heat generation on the brake resistor

Wiring

None

Connection to external brake resistor

required

Energy balance

Conservation of electrical energy

Electrical energy is converted into heat

Cost factor

Control electronics, inverter

Control electronics, switching transistor,

braking resistor, mounting, wiring

EMC requirements

Minor

Screened leads to braking resistor

4.2.3

Serial interfaces

The axis modules can be parameterized using a PC, through the integral standard RS-232 interfacein the power supply module. The RS-232 interface is used for communication between twocommunication units, e.g. PC and axis module.The RS-232 interface can be made busable in combination with the RS-485 interface, which is alsointegrated in the power supply module. This allows up to 31 physical or 59 logical axes, which areinterconnected via the RS-485 interface of their power modules to set parameters through theRS-232. Each axis only needs its individual address.MD0162AE

RS-232

ON24 V

ON24 V

T R IP

ON2 4V

T R IPS tat e/A dr.

T R IP

X 01

X311

6X 0 2

X2 1 X 32

X 01

X0

RS-485

X3 1

X3 1

6X 0 2

X 21 X3 2

X 21 X3 2

X0

X0

12

X 0

S1

X 01

S11

X31

X31

6X 0 2

X 21 X3 2

X2 1 X 32

X0

X0

12

S tat e/ A dr.1

S11

12

S tate/ A dr.1

S1

X 0

State/Adr.

S tat e/ Ad r.1

S1

12

X 0

12

RS-485

Fig. 37: Communication via serial interfaces

Drive Engineering - Practical Implementation - Volume 7

39

The servo controller

4.2.4

Electronics supply

The power supply module contains a central

switch-mode power supply (SMPS) for supplyingthe electronics. This produces a DC voltage of 24 Vfrom the DC link voltage VDC link. This voltage isrequired for supplying the monitoring electronics.At the same time, all axis modules connected tothe power supply module are fed with this voltagefrom the 24 V bus.The power supply module also offers the option ofconnecting an external 24 V supply. The controlelectronics remain operative by this externalvoltage, i.e. rotor position information and errormessages are retained, even if the main voltagesupply is interrupted. This is important, inparticular for operating a drive with positioningcontrol, because no further reference travel isneeded if the main voltage fails.

Power supply module

Axis module

SMPS

external 24 VMD0163AE

Fig. 38: 24 V supply

Also, the external 24 V power supply enables the user to parameterize the axis modules if the DC linkis not energized.4.3

Axis module

The axis modules are connected to the DC link and the protective earth conductor by means ofbusbars. A separate 24V bus is used for the control electronics power supply. A data bus is installedon the unit underside to enable a PC or a higher-level control (PLC) to communicate with theconnected axis modules (this bus is not directly accessible to the user).

L1L2L3Linechoke

M3~MotorOvervoltageprotection

Rectifier

RS-232

DC link

Brakechopper

Resolver

Inverter

EvaluationMonitoringsection

RS-485

24 V ext.

ControlOptions

Switch-modepower supply24 VVDC link

Power supply module

Switch-modepower supply24/15/+5V24VAxis moduleMD0196AE

Fig. 39: Block circuit diagram of a modular servo controller

The axis modules can be operated under speed control or torque control. They supply sinusoidaloutput currents, so that precise, true running with minimum torque ripple is ensured even at lowoutput speeds. The sinusoidal output currents minimize additional motor losses and ensure goodutilization of the motor power.

40

Drive Engineering - Practical Implementation - Volume 7

The servo controller

The axis modules and option pcbs parameters are set from a PC via the standard RS-232 interfaceor from a PLC via the RS-485 interface.Optional bus interfaces are also available for parameter adjustment.4.3.1

Design of the axis module

In the axis module, the functions of control, evaluation (resolver evaluation, current controller) andoptions (additional terminals, positioning control) are implemented in modular design. The advantageof this card-rack system is that other control procedures (e.g. V/f or flux vector control) can be usedby simply exchanging the control pcb and/or the evaluation pcb.Figure 41 shows the interrelationships in the axis module. The functions of the individual blocks areshown in the next sections and in the description of the control structure.

+VDC linkControl signals

Inverter

M3

X1117X12 State/Adr11S11 68X31X1311

-VDC linkEvaluation pcbCurentcontroller

712 7X14 X21 X32

R/D converter

X0

Encodersimulationnsetp.E/A

Processor pcbOpen and closedloop control

AdditionalZusatzterminalsklemmen

CPUMOVIDYN

Positioningcontrol

CPU / IPOSMOVIDYN

I controllerResolver

MonitoringCommunication

Option pcbAdditional I/O terminals

Fieldbusconnection

Positioning controlFieldbus connection

MD0181AE

Fig. 40: Interrelationships in the axis module

Drive Engineering - Practical Implementation - Volume 7

MD0180BE

Fig. 41: Modular design

41

The servo controller

The power inverterThe power inverter is supplied via the DC link voltage VDC link. The power transistors are switched bythe associated triggering circuit so that a pulse-modulated voltage is present at the output of the axismodule and thus, at the motor. The pulse width is determined by the current controller output. Thispulse width-modulated voltage produces a current in the motor which is almost sinusoidal becauseof the motor and cable inductances.A diode is connected in parallel to each power transistor. These free-wheeling diodes preventself-induced voltages from damaging the power inverter. These occur with inductive output loads atthe moment of switching. The diodes feed the stored energy back to the input of the power inverter.They also exchange reactive energy between the motor and the inverter.

M3~

MD0182AE

Fig. 42: Power inverter

Evaluation pcb functions

The evaluation pcb includes the following features:

resolver evaluation

encoder simulation

current control

The resolver evaluation and the encoder simulation have already been described in the precedingsections.The current controller is of analogue design. The current controller is set at the factory, and its settingmatched to the connectable motors.Control pcb functionsThe control pcb carries the microprocessor and its peripherals. The main functions of themicroprocessor are:

42

speed control with great flexibility

hold control

an internal positioning control (option)

extensive monitoring functions of system variables, inputs/outputs and control functions

communication between microprocessor, evaluation and option pcbs through the backplanebus

communication with other axis module(s)

Drive Engineering - Practical Implementation - Volume 7

The servo controller

4.3.2 OptionsAIO11 terminal expansionThe AIO11 option expands the control and monitoring possibilities of the axis modules with additionaldigital and analogue inputs and outputs and a serial interface.API/APA and IPOS positioning controlsPositioning controls are a simple means of implementing motional sequences, precise positioningand holding of a position (position control).The advantages offered by these option pcbs which use different feedback systems are:

reduced space requirement in the switch cabinet

voltage supply by servo controller

digital setpoint input for the speed

direct I/O control signal access

With IPOS integrated positioning the option slot remains free. This allows for the use of additionaloptions (i. e. PROFIBUS, INTERBUS-S, CAN-BUS).Fieldbus interfacesInterface pcbs are available for standardized fieldbus systems widely used in automationenvironments (e.g. PROFIBUS, INTERBUS-S, CAN-BUS).The fieldbus pcbs plug into the free option pcb slot. Through the fieldbus control signals, processdata and parameter values can be transmitted between the higher-level control system and the servocontroller.

Drive Engineering - Practical Implementation - Volume 7

43

Control structure /Modes of operation5

Control structure / Modes of operation

Electrical servo drives are used for position control in many types of application. To achieve a goodcontrol response, the position controller itself has an inner speed controller and a current controller.PositioncontrollerLsetp

Lact

Speedcontrollernsetp

CurrentcontrollerI

Isetp

nact

Iact

PWM

Gear unit

Load

ResolverR/D converter

MD0183AXAbsolute encoderMD0183AE

Fig. 43: Servo system control structure

The external position setpoint is the reference variable used by the servo controller. The error betweenposition setpoint and position actual value is the input for the position controller, which outputs therelevant setpoint speed nsetp for the motor.The speed setpoint and actual values are compared in the lower-level speed controller. The error issubject to PI control in the speed controller.The output signal from the speed controller is the current setpoint and is routed to a limiter circuitto protect the motor and inverter. The output signal from the limiter circuit itself becomes the setpointvalue for the current controller. The current actual values are converted into a DC signal by a rectifiercircuit. The current controller compares the setpoints and actual values and uses a pulse widthmodulator (PWM)to generate the control signals that are routed to the control stages of the individualpower transistors of the inverter.With the exception of current control, all open and closed-loop control and monitoring functions arehandled by a microcontroller. Current control is designed as an analogue circuit to satisfy speedrequirements.

5.1

Current controller

The current controller is designed as a PI controller. The input variable is the difference between thecurrent setpoint and actual values for a motor phase; the output is the control voltage for the pulsewidth modulator. This uses a sinusoidal-delta comparison to generate a pulse-width modulatedvoltage for controlling the inverter.The current actual value is measured on the inverter output using a DC instrument transformer andpassed to the comparator on the input of the current controller.The current controller is the innermost control loop of the servo controller and must therefore respondvery quickly, as this will determine the speed of all the higher-level controllers.

44

Drive Engineering - Practical Implementation - Volume 7

Control structure /Modes of operation

A current limiter is situated upstream of the current controller. This limiter limits the setpoint currentto a predetermined maximum value.The maximum current is determined by the three possible factors listed below:

thermal model (protects the output stage at low frequencies)

limiting by internal parameters

limiting by external inputs

The following block diagram shows the current controller and the three factors that determine thecurrent limit:Motor currentThermalmodel

Fig. 44: Current controller with current limiting

Limiting of the setpoint current using the thermal model is only effective at frequencies of up to 1.5 Hz.Within this frequency range, the current has to be limited as the conduction interval within the outputstage will otherwise be too long, with the result that the output stage will become too warm.Under normal operating conditions, the output stage can handle up to 150 % of the continuous ratedcurrent. At frequencies lower than 1.5 Hz, this threshold sinks to about 100 % IN. If the motor isbeing driven at less than 1.5 Hz, a current greater than 100 % IN is only permitted for a short time.If this permitted period is exceeded, the maximum current value is reduced to 100 %.One of the reasons for limiting the current through an internal parameter is to protect the motor. Theparameter is entered in the controller menu, depending on the maximum motor current. It can havea maximum value in the range 45 to 150 % IN.Limiting the current through an external input is only possible with the AI011 option pcb.The input signal is a voltage from 0 to 10 V. This voltage is set in proportion to the maximum voltagein this range. The quotient of the two voltage values is multiplied by the set maximum current value.The maximum current defined externally can therefore only be less than or, at the most, equal to theone defined internally.It is always the limit with the lowest value that is effective.

Drive Engineering - Practical Implementation - Volume 7

45

Control structure /Modes of operationExample: The maximum motor current (3 I0) is 110 % of the rated current of the servo controller.The motor is being operated at a frequency greater than 1.5 Hz.The requirements for external input of the maximum current are given and a voltage of 8 V is presenton the analogue input for external current input.

Limiting using the thermal model has no effect as the frequency of the motor is not within theeffective range for the model (< 1.5 Hz).

The internal parameter is programmed to 100 % of the rated current as this corresponds to themaximum permitted motor current.

The external value for the maximum current is entered as 8 V. In proportion to the maximumpossible voltage on this input of 10 V, this gives a factor of 0.8 (= 80 %). The internally definedmaximum current is multiplied by this factor.0.8 110 % IN = 88 % IN

The smallest maximum current value specified is therefore the current value of 88 % IN on theanalogue input.5.2

Speed controller

To ensure that the speed control has the required wide control range, even very low speeds still needto be detected accurately, a high-resolution rotor-position encoder and an extremely short samplingtime are required. This in turn demands a high processing speed and hence a particularly powerfulprocessor.nsetp

Rampgenerator

Setpoint nsetpnfilter- nact

Speedcontroller

Isetp

Actual valuefilternactMD0189BE

Fig. 45: Speed controller

The speed controller is superimposed on the current controller. It obtains the setpoint speed through:

positioning control

analogue input

fieldbus interface

serial interface

The speed controller is designed as a PID controller. All three controller components can be setseparately. In most applications the D component is set to zero because of its difficult adjustmentand optimization in order to prevent possible overshoot of the drive.

46

Drive Engineering - Practical Implementation - Volume 7

Control structure /Modes of operation5.2.1

Speed filter

Speed setpoint filter

Speed setpoint filters are necessary because:

analogue speed setpoints often contain errors

the speed setpoint value of the higher-level positioning control is staircased because of thecontrols cycle time.

The diagrams below show the curve of the speed setpoint and the torque at the motor without filterand in comparison with the ideal curve of the two variables.without filter

desired condition (with filter)

nsetp

nsetp

MMotor

MMotor

tMD0191AE

Fig. 46: Speed setpoint filter

The diagram of the setpoint speed without filter shows a staircased signal of a positioning controlwhich causes a pulse-type curve of the motor torque. In contrast, a continuous curve of the setpointspeed (and thus also of the motor torque) can be seen with the use of a speed setpoint filter.Speed actual-value filterSpeed actual-value filters are needed to filter out noise. This causes difficulties, particularly in therange of lower speeds.The time constant of a filter must be chosen so that the dynamic response of the drive is not restricted.If the time constant of the filter is too high, the system is slowed down and loses some of its dynamics.

Drive Engineering - Practical Implementation - Volume 7

47

Control structure /Modes of operation5.2.2

Speed controller with feedforward

The aim of using a feedforward is a better controlled acceleration (improving the control response).The additional P component of the feedforward makes for faster completion of the accelerationprocess. Since the feedforward is only effective within a certain range, this has no effect on thesystems control response in normal operation nor on its response to disturbances.Speed control with feedforward is used if acceleration is to be completed fast. The feedforward isconnected in parallel with the speed controller.Feedforwardlimit

Feedforward

Feedforwardfilter

nsetp

Rampgenerator

Setpoint nsetp'+filter-

FeedforwardP gain

+Speedcontroller

Isetp

Actual valuefilternactMD0190BE

Fig. 47: Speed controller with feedforward

The speed setpoint supplied by the ramp generator is filtered in the feedforward filter. The filteredsignal is routed to a differentiator circuit. The magnitude of the differentiator output depends on thechange of speed over time. If the value exceeds the trigger threshold of the feedforward, it is fed toa P element. The gain of the feedforward can be set at this P element. The output of the P element isin turn routed to the input of the current limiter circuit.If the value falls below the trigger level again, the feedforward is inhibited and the speed controlleris effective on its own again.If a drive is operated without feedforward, a considerably higher I component is developed in thespeed controller, which results in drive overshoot.5.3

Position controller

The position controller is implemented as pure proportional

controller. An integral component would result in impermissibledrive overshoot when the target position is approached. Theintegral component of the lower-level speed controller makes surethat there is no constant position error (e.g. under load).

Lsetp

L-

Lact

Positioncontroller

MD0192CE

Fig. 48: Position controller

48

Drive Engineering - Practical Implementation - Volume 7

Control structure /Modes of operation

Hold controlThe hold controller is a variation of the position controller. It ensures that the original position isretained without the need for an external positioning control even if disturbances occur (e.g. loadingor unloading of a hoist). The hold control is a special function which can be activated via a binaryinput. If the input goes high (logical 1), the setpoint value n = 0 (zero speed) is given to the speedcontroller. The drive ramps down the active setpoint ramps to zero speed. When zero speed is firstdetected by the speed detection system, the position actual value is stored as position setpoint value.At the same time, the output of the position controller is routed to the input of the speed controller.The system is now position-controlled at the position which has been stored as the setpoint value.Lact

L PositioncontrollerLsetp

nsetp

Rampgenerator

nsetp= 0

Zero rampsAdjustable rampsn=0firstreached

nact

Save targetposition

LactMD0193BE

Fig. 49: Hold control

5.4

Operating modes

Servo controllers are used in two different operating modes:

speed control

torque control

5.4.1

Speed control

Speed control consists of a speed control loop with upstream speed limiting. Speed limiting limitsthe setpoint value coming from the setpoint source to a maximum speed. If the drive reaches thesetpoint speed or the maximum speed, it will stay at this speed and thus operate under speed control.If the load on the motor is too great, the drive will reach the current limit before the motor attains thespecified speed. If the load increases still further, the motor may come to a standstill. An error willonly be detected if the speed monitoring function is active or when the temperature monitoring ofthe heat sink trips.

Drive Engineering - Practical Implementation - Volume 7

49

50

AnalogueinputPC interfaceFielbus option

Amplifier

Offset Gradient of V/f

characteristicSetpointsourceSpeedlimit

RampgeneratorSetpointfilter

Filterfeedforward

Feedforward

Motor

P gainfeedforward

Speedcontroller

Resolver

Speeddetection

Actual valuefilter

Limitfeedforward

Inverter

Maximumcurrent limit

Isetp

Currentcontroller

Iact-

5Control structure /Modes of operation

MD0195BE

Fig. 50: Speed control

Drive Engineering - Practical Implementation - Volume 7

Control structure /Modes of operation5.4.2

Torque control

Pure torque control of servo drives is mainly used in what is known as current slaving inmaster-slave applications.The present current actual value of the master is passed to the slave as an input signal (setpoint).Both drives must be mechanically connected to each other (e.g. by a shaft). The slave will then deliverthe same torque as the master and the load will be shared between the two drives.Another application of torque control can be found, for instance, in winder drives.In torque control, the speed controller is overridden. This means that the controller is always fullyunder control. The current limit is given by the setpoint torque, i.e. the current setpoint. The directionof rotation (nmax CW or nmax CCW input) is determined by the sign of the torque setpoint value.When operating under torque-controlled conditions, the motor will not normally reach the specifiedspeed value nmax as the current limit is active. The current value will then correspond to the setpointcurrent, i.e. the setpoint torque is reached. The motor is thus torque-controlled.If the load torque is not sufficient to reach the setpoint current value, the motor accelerates up to themaximum speed nmax.

Drive Engineering - Practical Implementation - Volume 7

51

The gear unit

The gear unit

In its function as a converter of torque and speed, the gear unit is the central component of the gearedmotor.6.1

Demands of gear units in servo technology

low inherent mass moment of inertia

low circumferential backlash

high torsional rigidity

high efficiency

precision-balanced

A low-inertia gear unit is essential for high-dynamic drives. Whenever fast acceleration of a drive isa major requirement, use of a dynamic gear unit with as high an efficiency as possible is almostinevitable.Extremely low circumferential backlash and high torsional rigidity are required when using apositioning control because otherwise relatively high angle errors occur which make accuratepositioning impossible.

6.2

General overview of gear units

Gear unit types

PositiveWheel gears

Belt drives

Helical gear

Toothed belt

Planetary gearBevel gear

Non-positiveChain gearsRoller chainInverted-tooth chain

not suitable for automation

axes, as the system-inherentslip does not permit accuratetransmission of the positionMD0202AE

Fig. 51: Overview of gear units

A distinction is made, according to the direction of the power flow, between coaxial or parallel shaftgear units and right-angle gear units. With coaxial and parallel-shaft gear units, the input and outputshafts are in the same plane, and the power flow is linear. With right-angle gear units, the input andoutput shafts are at 90 to one another and the power flow is turned through a right angle.

52

Drive Engineering - Practical Implementation - Volume 7

The gear unit

In the following, the most common gear units used in servo technology are described.6.2.1

Helical gear units

Helical gear units are the most commonly used gear unit. They are inexpensive to produce, requireno complicated machine tools for manufacture and allow the use of controlled production methods.Their simple and rugged design suits most applications.In helical gear units the input and output shafts run parallel to each other. This makes the overalldrive short and slim, i.e. ideally suited for applications where space is tight. In this parallel shaftarrangement the output shaft is usually designed as hollow shaft, an advantage in particular in traveldrives where the straight-through axle can transmit the force synchronously onto both drive wheels.

Helical-bevel gear units

Particularly compact drive solutions can be implemented with helical-bevel gear units where thepower flow is turned through a right angle.In helical-bevel gear units output shafts can be implemented as hollow shaft or as solid shafts.6.3

Comparison of different gear unit types for servo technology

Helical gear units

Planetary gear units

Helical-bevel gear units

Power density

medium

high

medium

Gear ratio per characteristic

stage

smallapprox. i = 1 ... 8

mediumapprox. i = 4 ... 10

smallapprox. i = 1 ... 6

Circumferential backlash

medium

small to very small

medium

Torsional angle (at output)

= 12...18

= 3...10

= 12...18

Torsional rigidity

medium

high

medium

Noise

medium

low

medium

Drive Engineering - Practical Implementation - Volume 7

53

Use in an industrialenvironment7

Use in an industrial environment

7.1

Mains conditions

In the case of industrial mains, a sine-wave voltage is assumed. Unstable conditions normally haveno effect. The controllers can be used with most types of mains (TN, TT, etc.).Voltage fluctuations can affect how the drive works. Within the rated voltage range, the drive willfunction normally. If the range is exceeded, the drive will have to shut down to prevent damageoccurring. If the voltage is too low, the motor will no longer deliver the rated values specified in thetechnical data. The mains voltage frequency is of minor significance.The use of line chokes and protective circuits make the servo controllers immune to voltage spikesthat can, for example, occur in reactive power compensation equipment systems without line chokes.

7.2

Notes on the motor

How to select a motor is shown in the project planning example. The motor and servo controller mustbe matched to each other.Servo motors are usually fan-cooled. As heat dissipation is through convection, the colour andcleanliness of the motor are important.The motors are protected to IP65 as standard. The continuous torque can be increased by a factorof 1.6 through the use of a forced cooling fan.

7.3

Cabling

The type of cables and how they are laid is very important in servo drive applications. The cablesmust be dimensioned to suit the current flowing through them and thus ensure that the voltage dropis kept to the specified value. Refer to the applicable regulations for further information on cabledimensioning.The laying of the cables, especially when using a cable duct or rack, requires a great deal of care. Theeffect of EMC will be reduced if power cables and electronic leads are laid separately. Screened cablesare excellent at preventing electromagnetic interference in the system.

7.4

Electromagnetic compatibility (EMC)

The EMC of the components of a system and of the system itself is extremely important. The EMCDirective lays down the permitted conditions. This defines not only the levels of emitted interference,but also the immunity to interference. All SEW servo controllers are interference-suppressed, whichmeans they can be used in industrial environments. To suppress interference, the use of screenedcables and a mains input filter is recommended.The resolver lead in particular must be screened. How the screen is to be earthed depends on severalfactors. For further instructions please refer to the applicable documentation.

Fig. 52: EMC-compliant wiring in residential areas

Drive Engineering - Practical Implementation - Volume 7

55

Use in an industrialenvironment7.5

Interfaces to the environment

Several channels are available for control of and feedback from the servo controller. Feedback signalsfrom the servo controller are routed to higher-level functional units such as a PLC or an externalpositioning control. It is important that the applicable interface standards be adhered to.Interface

Level

Binary inputs

1" +13V ... 24V ... 30.2V

Analogue inputs

-10V ... +10V

RS-232

to RS-232 standard

RS-485

to RS-485 standard

Encoder simulation

RS-422 TTL standard

Fieldbus

to the applicable standard (Profibus, INTERBUS-S, CAN bus)

The powerful binary outputs can be used to drive commercial relays or small power contactors. Theadvantage of a relay driver output is that the electronics need not be exchanged together with therelay when the relay fails.The inputs are electrically isolated with optocouplers, offering maximum noise immunity. They canbe controlled directly from a PLC etc.The controller parameters can be easily set and commissioned through the standard RS-232 PCinterface. The following diagram shows the flexible use of a controller in an industrial environment.

PLC

Positioningcontrol

Sensor / actuator

Servo controller

00040AEN

Fig. 53: Interfaces

56

Digital inputs/outputs:

Freely programmable with specified functions

Analogue inputs:

Setpoint input for speed, torque, maximum current

Analogue outputs:

Output of process values

Serial interface:

RS-232/RS-485 for connection of a PC, process interfacing

Encoder simulation:

Incremental position signal for internal/external position controller

Fieldbus:

Process interfacing

Drive Engineering - Practical Implementation - Volume 7

Use in an industrialenvironment

Figure 53 shows possible system configurations as seen from the servo controller:

The direct link between servo controller and sensor-actuator level (limit switches, pressure marks,sensors etc.) is the simplest type of application. Simple tasks can be handled without connectionto a PLC or positioning control.

With more complex systems there is normally a higher-level control (PLC). Communication isnormally via the I/O level of the PLC and the servo controller. The PLC must have an analogueoutput for analogue setpoint processing. Communication is also possible via serial interface orfieldbus, e.g. for visualization of data, parameter adjustment, process data exchange etc.The sensor-actuator level can be connected directly or through the PLC.

Where positioning functions have to be performed, positioning modules are frequently used asaccessory modules to the PLC or as standalone systems. In addition to the connection possibilitieslisted in point 2, the encoder simulation of the servo controller is generally used to provide theincremental position actual value signal for the positioning module.

Apart from being integrated into the PLC, the positioning control can also be implemented asoption pcb or as software in the servo controller. In this case setpoint input to the servo controlleris digital. Connections can be made via the I/O level of the servo controller, a serial interface orthe fieldbus.

7.6

Definitions of process interfacing

7.6.1 Classification by drive configuration

Single-axis applications:In terms of control design this application is the least demanding for the higher-level control.Typical is the use of a positioning control with digital input/outputs for monitoring the drive.

The working process determines the control complexity in multi-axis applications. The dependenceand accuracy of movement of electrically coupled drives are decisive in the choice of a suitablemulti-axis control.

Drive Engineering - Practical Implementation - Volume 7

57

Use in an industrialenvironment7.6.2

Classification by setpoint sources for the speed controller

Setpoint input without process interfacing (control):

- internal digital setpoint input in MD_SHELL panel mode (Test/Commissioning).- analogue speed or torque control by means of potentiometer.- analogue speed control with position or time-dependent setpoint selection.

Classification by type and mounting location of the position encoder

When using a positioning control the position actual value must be detected by an encoder. The pointof measurement depends on the influence of disturbances, the resolution of the encoder and therequired accuracy. Compatibility with the positioning control must also be considered when choosingthe encoder. For many applications the encoder simulation integrated in the servo controller is used.

Position encoder:- incremental encoder simulation of the resolver signal- external incremental encoder at a working process location (material to be conveyed,conveyor belt, etc.)- absolute encoder at the motor

7.7

Ambient conditions

The ambient conditions must be looked at separately for the motor and the servo controller. Themaximum ambient temperatures for the motor and the servo controller are:Motor:Servo controller

-25C...40C

with 100% M0

max. q = 60C with 75% M0

0C...45C

with 100% PN

max. q = 60C with 55% PN

For further details of the permissible ambient conditions see the notes on project planning.7.8

Commissioning and controller optimization

Commissioning and controller optimization are nowadays performed similarly to a PLC, CNC etc.,using a PC and the associated software. The software must be simple, clear and user-friendly. Inaddition, there are operating controls inside the unit.Prior to commissioning the servo drive system the mounted system components must be comparedagainst the project planning data for correctness.If the cables are laid and wired correctly (screening), commissioning of the drive can be started.

58

Drive Engineering - Practical Implementation - Volume 7

Use in an industrialenvironment7.8.1

Controller setting with the MD_SHELL user interface

The user interface described here enables the user to carry out a rapid first commissioning. Thisincludes the basic setting of the speed controller, calculated from project-specific data, in the userinterface.

Select the Commissioning menu item of the Parameters menu

Enter the requested data

motor frame size

rated speed brake damping of speed control loop stiffness of the speed control loop positioning control time interval drive with / without backlash load moment of inertia reflected to the motor shaft shortest required ramp timeBy pressing the [F2] key all necessary parameters are calculated and the limit values for the givendrive set. The drive can be commissioned with the basic setting of the speed controller displayed.

00042AEN

Fig. 54: Commissioning menu

The basic setting normally gives satisfactory results, although the following aids are available if furtheroptimization is required:

Use of the MD_SCOPE software for process data visualization which offers the function of adigital storage oscilloscope. Setpoints and actual values etc. can be displayed on a PC monitorscreen as a function of time, they can be stored and printed out. At the same time, controlparameters can be changed without having to change to the user interface.

Drive Engineering - Practical Implementation - Volume 7

59

Use in an industrialenvironment

The control parameters can be optimized without a utility program, using the AIO11 option andan oscilloscope. To do this, the analogue outputs on the option must be programmedaccordingly.

7.8.2

Controller optimization using MD_SCOPE

00043AEN

Fig. 55: Process data tracing with MD_SCOPE

Fig. 55 shows the trace of the selected measured values (actual speed, current setpoint, rampgenerator input and output) for adjustment of the parameters as calculated by the user interface. Thedrive rapidly attains the setpoint speed, overshoots once, and reaches the setpoint value relativelyquickly.The parameters Damping and Stiffness allow all parameters of the control loop to be adjustedfor smooth control response.

Fig. 56: Project planning flowchart

Drive Engineering - Practical Implementation - Volume 7

61

Project planning8.2

Project planning example

For a three-axis gantry application the servo drives and the appropriate power electronics shall beselected. The axes of the gantry system shall be referred to as X, Y, Z, reflecting their location inspace.X axisTravel axis driving two toothed belts via a shaft. The toothed belts move the two drive units Y and Zin the plane.Y axisTravel axis moving the Z axis via a toothed belt. The direction of travel is at 90 relative to the X axisin the plane.Z axisHoist axis with power transmission by means of a gear rack.

YZXYZ

X00059AEN

Fig. 57: Arrangement of the axes

The design calculations shall be done separately for each axis. All calculations are based on linearacceleration and deceleration. Further requirements are the use of modular components and of abraking resistor. Positioning is controlled by a higher-level PLC.Note:* refers to load data reflected to the drive side.8.3

Constant travel, static load

Load torques in the travel cycle

Acceleration phase

M1 = Mstat + M dyn1 = 440.42 Nm + 86.41Nm = 526.83 Nm

Uniform motion

M 2 = Mstat = 86.41Nm

Deceleration phase

M 3 = Mstat + M dyn2 = 86.41Nm 356.74 Nm = 270.33 Nm

The maximum torque demand M1 determines the required Mamax of the gear unit and consequently the gear unit size.The selected gear unit is a PSF 701, i = 10, Mamax = 800 Nm with an EB10" curved-tooth coupling. The latter was chosen to satisfy the application requirement for separability of motor and gear unit.

8.3.4

Motor-reflected torques and mass moments of inertia

When determining the motor-reflected torques the efficiencies and moments of inertia of both the gear unit and the motormust be taken into account.Gear unit data (see PSF Planetary Gear Units Catalogue)single - stage planetary gear unitPSF 701 / EB 10:

d) Mrms < Mperm

All requirements are met.

66

Drive Engineering - Practical Implementation - Volume 7

Project planning8.3.7

Selection and design calculation of the servo controller components

The worked example below gives the selection details of an individual drive (single-axis drive). Afterconcluding the design calculations for the X, Y and Z axes, the supply for all three drives togethershall be determined in comparison.Axis moduleThe axis module must satisfy the following selection criteria:a) I N >

Imax1.5

This condition follows from the axis modules' ability to supply 1.5 times the rated current.Imax =

M1MotM0

I0 =

74.58 Nm 24 A = 51.14 A35 Nm

IN > 34.1A

b) IN > I

The current mean value I is a measure of the axis modules' thermal load rating.I=I=

b) PBRC max > PB max

c) PDC link N > P

PN cdf > Pregen

The cdf rating of the braking resistor must be greater than the mean regenerative braking power.

Pregen =

PBmax 11902 W== 5951W22

cyclic duration factor cdf BW [%] =

tatZ

100 % =

0.25 s 100 % = 11.9 %2.1s

b) Checking the combinability of braking resistor and supply component.

Selected braking resistor:

BW 018-015

Rated data:

PN = ,5 kWPN at 12 % cdf = 9 kW

Requirements:

68

a) PN cdf > Pregen

9 kW > 6 kW

met

b) braking resistor permissible as per catalogue

met

Drive Engineering - Practical Implementation - Volume 7

Project planning

Heat sinkThe following criteria must be considered when selecting the heat sink:a) The total width of all modules added together. Care must be taken to ensure that the modules are not mounted overthe joint between two heat sinks.b) The maximum heat sink temperature KKmax (80 C) may not be exceeded (taking account of the ambient temperature).Required width:

MBP 51A-027-503-00:MAS 51A-060-503-00:

4 TE4 TE

Power losses to be dissipated by the heat sink:

PKK =PVSNT =PVL0 =PVLX =k=I=

heat sink rating

power losses of the switch-mode power supply in the power supply modulepower losses of the power supply modulepower losses of the axis modulenumber of axis modulescurrent mean value

PKK = 21 PVSNT + PVL0 + PVLX

Note: For an explanation of the constants used please refer to the Appendix.Selected heat sink:

DKS 09

Rated data:

Rth =Width =

0.17 K/W9 TE

Calculating the temperature rise when ambient temperature amb = 30 C

K 196.5 W = 33.4 K = R th PKK = 0.17 W

KK = amb = (30 + 33.4) C = 63.4 C

Requirements:a)

9 TE > 8 TE

b)

KK < KKmax;

met63.4 C < 80 C

met

Drive Engineering - Practical Implementation - Volume 7

69

Project planningData bus cable and line chokeThe length of the data bus cable depends on the number of axis modules connected to the power supply module:1 MPB + 1 MAS DBK01The appropriate line choke can be found in the assignment table in the installation and operating instructions.Power supply module:

MPB 51A-027-503-00

Line choke:

ND 045-013

Rated data:

IND = 45 ALH = 0.1 mH

Overview of selected components

An overview of selected components shall first be given for the single-axis drive; the selection for the complete examplewith X, Y and Z axes shall be listed at the project planning example.

Constant travel, static load

Load torques in the travel cycle

Acceleration phase

M1 = M stat + M dyn1 = 12.59 Nm + 128.33 Nm = 140.92 Nm

Uniform motion

M 2 = M stat = 12.59 Nm

Deceleration phase

M 3 = M stat + M dyn2 = 12.59 Nm 103.95 Nm = 91.36 Nm

The maximum torque demand M1 determines the required Mamax of the gear unit and consequently the gear unit size.The selected gear unit is a PSF 401, i = 10, Mamax = 150 Nm with an EB09" curved-tooth coupling. The latter was chosen to satisfy the application requirement for separability of motor and gear unit.

72

Drive Engineering - Practical Implementation - Volume 7

Project planning8.4.4

Motor-reflected torques and mass moments of inertia

When determining the motor-reflected torques the efficiencies and moments of inertia of both the gear unit and the motormust be taken into account.Gear unit data (see PSF Planetary Gear Units Catalogue)G = 0.97JG* = 5.76 104 kgm2 (reflected to the motor shaft)

Load torques in the travel cycle (reflected to the motor)

Additional torques for the gear unit moment of inertia reflected to the motorM 1G* =

JG * 2 n *s t60 mina

5,76 104 kgm 2 2 2728 min1

= 0.68 Nms 0.25 s 0.9760 min

There is no M2G for M2 sin ce there is no change in speed.

M 3G* =

JG * 2 n * Gs t60 mina

5.76 104 kgm 2 2 2728 min1 0.97

= 0.64 Nms 0.25 s60 min

Mass moment of inertia of translatory load movement

JL* = m L

8.4.5

FG 60 IJ FG v IJH 2 K H n* Ksmin

max

= 132 kg

FG 60 IJ FG 2.5 IJH 2 K H 2728 min Ksmin

ms

= 0.0101kgm 2

Motor selection and rms torque

The following conditions must be verified for motor selection:

a) k j =

Jext *< 10Jmot

(mass moment of inertia)

To ensure satisfactory control response the ratio of external moment of inertia to motor moment of inertia should beless than 10.b) Mmax* < 3 M0The maximum dynamic load on the drive may not exceed three times the motor rated torque.c) Mrms < MpermThe rms which is effective over the entire travel cycle may not exceed the motor rated torque taking account of the motor characteristic (Mperm based M0 and motor characteristic).d) n* 0.9 nNThe maximum expected speed should be 90 % of the motor rated speed to provide for a control reserve of approx. 10 %.Now a motor is selected initially without taking account of the motor moment of inertia. The selection must then be confirmed by way of recalculation using the actual motor moment of inertia.Jext * = J L * + JG* = 0.0101kgm 2 + 0.0006 kgm 2 = 0.0107 kgm 2Mmax * = M1 * +M1G* = 14.53 Nm + 0.68 Nm = 15.21Nm

d) Mrms < Mperm

All requirements are met.

74

Drive Engineering - Practical Implementation - Volume 7

Project planning8.4.7

Selection and design calculation of the servo controller components

At this point only the required axis module shall still be determined. For the supply module only the applicable criteriawill be specified. The power supply module will then be determined jointly for all three axes.Axis moduleThe axis module must satisfy the following selection criteria:a) I >N

Imax1.5

This condition follows from the axis modules' ability to supply 1.5 times the rated current.Imax =

Power supply module

The power supply module must satisfy the following selection criteria:a) PZ max Pmot max

The maximum DC link power rating must be greater than the maximum required power of the drive.Pmot max =

n * M 2Mot 2s60 min

2728 min1 17.96 Nm 2

= 5131Ws60 min

Drive Engineering - Practical Implementation - Volume 7

75

Project planningb) PBRCmax > PBmax

The braking power of the power supply module must be greater than the braking power of the drive.PBmax = n * M 3mot

22 L = 2728 min1 12.25 Nm 0.9 = 3150 Wss60 min60 min

c) PDC link N > P

The DC link power rating must be greater than the mean power of the drive.

FGH

P=

tZ

P=

1.3 s

1P2 max

ta +

M 2mot n * 2s60 min

t c + 21 PBmax t a

IJK

1 5131 W 0.25 s + 371.4 W 0.15 s + 1 3150 W 0.25 s

22

j = 839 W

Braking resistor duty cycle

t0.25 scdfBW [%] = a 100 % = 100 % = 19.23 %tZ1.3 sNormally the selection of power supply module, braking resistor, heat sink and line choke would follow at this point.Since these components shall be selected though for a multi-axis application with the MOVIDYN modular servo controllerthese components will only be selected after the design calculations for the Y, Y and Z axis have been completed.

M 3 = M dyn2 + M stat1 = 9.00 Nm + 10.90 Nm = 1.90 Nm

The maximum torque M1 determines the required Mamax of the gear unit and consequently the gear size.The selected gear unit is a PSF 301, i = 4, Mamax = 80 Nm with an EB04 curved-tooth coupling. The latter was chosento satisfy the requirement for separability of motor and gear unit.

8.5.4

Motor-reflected torques and mass moments of inertia

When determining the motor reflected torques, the efficiencies and moments of inertia of both the gear unit and the motormust be taken into account.Gear unit data (see PSF Planetary Gear Units Catalogue)single-stage planetary gear unit:PSF 301 / EB04

G = 0,97JG* = 2.3 104 kgm2 (reflected to the motor shaft)

Load torques in the travel cycle (reflected to the motor)

Acceleration phase M * = M 1 = 22.01Nm 1 = 5.67 Nm11Lifting0.97 4 iG

Uniform motionLifting

M 2* = M 2

11= 10.90 Nm = 2.81Nm0.97 4G i

1Deceleration phase M * = M 1 = 190= 0.49 Nm. Nm 33Lifting0.97 4 iG

Stan dstill

11M 4 * = M 2 = 10.90 Nm = 2.73 Nmi4

Acceleration phase M * = M 1 = 2.28 Nm 1 = 0.59 Nm

55Lowering i0.97 4G

Uniform motionLowering

0.97= 2.14 NmM 6 * = M 6 G = 8.83 Nm i4

Deceration phaseLowering

M 7* = M 7

Stan dstill

11M 8* = M 2 = 10.90 Nm = 2.73 Nmi4

G0.97= 17.83 Nm = 4.32 Nmi4

Additional torques for the gear unit moment of inertia reflected to the motor

Braking resistor duty cycle

cdfBW [%] =

2 t a + tc2 0.19 s + 0.336 s 100 % = 100 % = 25.6 %tZ2.8 s

Normally the selection of power supply module, braking resistor, heat sink and line choke would follow at this point. Sincethese components shall be selected though for a multi-axis application with the MOVIDYN modular servo controller thesecomponents will only be selected after the design calculations for the X, Y and Z axis have been completed.

Drive Engineering - Practical Implementation - Volume 7

83

Project planning8.6

Common supply of the X, Y and Z axes

When we take a look at the v/t diagram for all three axes, we see that all three axes must accelerateat the same time. The power supply module must be able to handle this worst case. As is evidentfrom the v/t diagrams, the deceleration phases do not coincide. However, in the event of an emergencystop all axes must decelerate at the same time.

X axisv

Y axisv

Z axisv

t [s]0

00221AEN

Fig. 61: v/t diagram for all three axes

For the design calculation a demand factor of 1 is used, i.e. all axes can accelerate and decelerate atthe same time. Therefore currents and torques are added up.

8.6.1

Power supply module

The power supply module must satisfy the following selection criteria:a) PDC link max Pmot maxThe maximum DC link power rating must be greater than the maximum required power of the drive.

The permissible cdf time is verified by approximation:

g b11.9 + 19.2 + 25.6g% = 18.9 %

b) Checking the combinability of braking resistor and supply components

Selected braking resistor:

BW 018-035

Rated data:

PN = 3.5 kWPN at 25 % cdf = 10.25 kW

Requirements:a) PN cdf > Pregen10.25 kW > 8.3 kW

met

b) braking resistor permissible as per catalogue

met

Heat sinkThe following criteria must be considered when selecting the heat sink:a) The total width of all modules added together. Care must be taken to ensure that the modules are not mounted overthe joint between two heat sinks.b) The maximum heat sink temperature KKmax (80 C) may not be exceeded (taking account of the ambient temperature).Required width:

KK09 = amb + 09 = 30 + 33.4 C = 63.4 C

PKK05 = PVLY + PVLZ = IY 14 W

KK05 = amb + 05 = 30 + 23.7 C = 53.7 C

Note: For an explanation of the constants used please refer to the Appendix.Requirements:a) 14 TE > 12 TEb) KK09 < KKmaxKK05 < KKmax

met63.4 C < 80 C53.7 C < 80 C

metmet

Data bus cable and line choke

The length of the data bus cable depends on the number of axis modules connected to the power supply module:1 MPB + 3 MAS DBK03The appropriate line choke can be found in the assignment table in the Installation and Operating Instructions.Power supply module: MPB 51A-0503-027-00Line choke:ND 045-013Rated data:IND = 45 ALH =0.1 mH

86

Drive Engineering - Practical Implementation - Volume 7

Project planning

Overview of selected components

Geared motors:

X axis: PSF 701 EB DY 112 LB

Y axis: PSF 401 EB DY 90 MBZ axis: PSF 301 EB DY 71 MLB

Axis modulel:

X axis: MAS 51A-060-503-00

Y axis: MAS 51A-010-503-00Z axis: MAS 51A-005-503-00

Power supply modulel: MPB 51A-027-503-00

Braking resistor:

BW 018-035

Heat sink:

DKS 09 und DKS 05

Line choke:

ND 045-013

Data bus cable:

DBK03

Software:

MD_SHELL

Drive Engineering - Practical Implementation - Volume 7

87

AppendixDetermining the power losses

Switch-modepower supply

1/2 PVSNT

1/2 PVSNT

Power supply module

Axis module axis 1

Axis module axis 2

Axis module axis 3

Power section

PVL0

PVS1

Signalelectronics

Power section

PVL1

PVS2

Signalelectronics

Power section

PVL2

PVS3

Signalelectronics

Power section

PVL3

00224AEN

Fig. 63: Power loss components

Power losses PVSNT of the switch-mode power supply in the power supply module:PVSNT = 12 W + 13 W k12 W = constant, power losses of the switch - mode power supply13 W = constant, power losses for every connected axis modulek = Number of connected axis modulesPower losses PVL0 of the power section in the power supply module:PVL0 = I 2 W/AI = mean value of the axis module current2 W/A = constant, power losses per ampPower losses PVL1/2/3 of the power section in the axis module:PVL1/2/3 = I 14 W/AI = mean value of the axis module current14 W/A = constant, power losses per ampPower losses PVS of the signal electronics in the axis module:PVS1/2/3 = 40 W k40 W = constant, power losses of the signal electronicsk = number of the joined axis modulesTotal power losses to be dissipated by the heat sink:PKK = 1/2 PVSNT + PVL0 + PVL1 + ...Power losses PSS in the switch cabinetHeat sink mounted externally:Heat sink mounted in the switch cabinet:

88

PSS = 1/2 PVSNT + PVS

PSS = 1/2 PVSNT + PVS + PKK

Drive Engineering - Practical Implementation - Volume 7

AppendixDimensioning of braking resistorsA servo controller with a brake chopoper requires a braking resistor to absorb the excess brakingenergy. The brake chopper is connected to the DC link circuit and switches on automatically if theDC link voltage VDC link reaches a certain level. The braking resistor which is connected to the brakechopper continuously absorbs energy from the DC link until the switch-off threshold of the DC linkvoltage is reached. During continuous braking the brake chopper switches on and off continuously(it chops).The resistance value () of the braking resistor is determined by the maximum permissible brakingcurrent of the brake chopper transistor. The permissible resistance value for each servo controllertype is given in the Technical Data.The design rating (100% cdf rating) of the braking resistor is given by the electrical braking powerwhich flows back into the servo controller after deduction of the losses (reverse efficiency ) in themachine, gearing and motor. Since the braking power is usually not continuous but for a limitedperiod only, this aspect can also be considered in the dimensioning of the braking resistor.In linear decelerations the braking power PB falls linearly with the braking time tB. This means thatthe peak braking power at the start of the braking phase is twice as high as the mean braking power.The resulting continuous regenerative power rating of the braking resistor (100 % cdf) for a singlebraking operation within a cycle time (repeat cycle time) can be determined from the braking power(for 100 % cdf) with the following nomogram:Cyclic duration factor of the braking resistor cdf [%]4

10

20

30

40

50

60

80

10010080

60

60

40

40

20

20

108

108

10.80.6

10.8

0.4

0.60.4

0.2

0.2

Continuous braking power (100% cdf) [kW]

Short-time braking power [kW]

10080

0.1

0.13

8 9 10

20

30

Cyclic duration factor of the braking resistor cdf [%]

40

50

60

70 80 90100

00223AENFig. 64: Determination of the short-time braking power for repeat cycle times (cycles 120 s) from the continuous resistorrating (= 100 % cdf). The lower curves (0.1 - 4 kW; see scale continuous rating on right) are valid for wire-woundtubular resistors, the upper curves /5/9/13/18 kW) for steel grid resistors.Example for the selection of a braking resistor:The required short-time braking power of 1 kW calls for a braking resistor with a continuous regenerative power ratingof 150 W at 10 % cdf.